1. Field of the Invention:
This invention is a further development in the field of direction selecting for ultrasonic imaging systems. In particular, the invention is concerned with the introduction of discontinuous delay values between the various elements of an array of transducers, and with an optimized switching arrangement for minimizing the number of electronic components required to provide the required delay values for steering and focusing of the ultrasonic imaging system.
2. Description of the Prior Art:
In a pulsed ultrasonic beam imaging apparatus, a particular scanning angle and focal distance for an array of electromechanical transducers can be obtained by pulsing each of the transducer elements of the array in a proper timing sequence, so that the acoustic pulses transmitted from each of the transducer elements all arrive at the desired focal point at the same instant in time. This principle is illustrated schematically in FIG. 1 where the transducer array 10 consists of an unspecified number of individual transducer elements, with the element on the left end being indicated by the reference number 11, the element on the right end being indicated by the reference number 20, and intermediate elements of the array being indicated by the reference numbers 12, . . . , 15, . . . , 19. The element 15 represents the transducer element disposed at or near the center of the array. In order to focus an acoustic pulse at the point P which is located at a distance from the transducer array 10, it is necessary that the pulses transmitted from the individual transducers all arrive at point P at the same time. Thus, the acoustic pulse from the left-end transducer 11 must travel to a point B on the pulse path to the focal point P before the right-end transducer element 20 is excited to emit an acoustic pulse. By delaying the electrical excitation of each of the transducer elements to the right of the end element 11 by an appropriate amount, it is possible to bring the pulses from all the transducer elements simultaneously to a focus at point P. The focal point P is identified by the focal distance f from the center element 15 to the point P, and by the angle A between the normal to the array at the center element 15 and the path from center element 15 to point P.
Similarly, in order to operate the transducer array 10 in a receiving mode so as to focus upon the point P as a source of reflected acoustic energy, it is necessary that, as a wave front reflected from the point P impinges in turn upon each of the transducer elements of the array, the electronic signals thereby generated by each of the transducer elements in succession be detected simultaneously by a receiver. For example, as seen in FIG. 1, a signal reflected from the point P will arrive simultaneously at the right-end transducer element 20 and at the point B on the path from point P to the left-end transducer element 11. Therefore, the electronic signal produced by the right-end transducer element 20 when operating in the receiving mode must be delayed during the time interval required for the acoustic wave front travelling along the path from point P to the left-end transducer element 11 to travel the distance from point B to element 11. The electronic signals generated by the intermediately disposed transducer elements of the array 10 must likewise be delayed by suitable intermediate time intervals before being combined so as to provide a coherent image of the point P.
Various techniques have been used in prior art imaging systems for obtaining coherent delays between the individual receiving elements of a transducer array in order to provide an electronic analog image of the source of reflected waves. One such prior art technique is shown in FIG. 2 where the transducer elements 11, 12, . . . , 20, representing an unspecified number of transducer elements, are arranged in a linear array with the left-end element being indicated by reference number 11 and the right-end element being indicated by the reference number 20. Each transducer element is coupled through a separate variable delay line 21, 22, . . . , 30, respectively, to a transmit/receive unit 31. The transmit/receive unit 31 is programmed to transmit electrical pulses to the individual transducer elements for conversion into acoustic pulses, and to receive electrical pulses generated in the individual transducer elements by reflected ultrasonic waves. The processing of the received signals by the the transmit/receive unit 31 occurs during the quiescent period between pulse transmissions. The particular delay value for each of the variable delay elements 21, 22, . . . , 30 is controlled by a controller 35, and is determined by the desired scanning angle for the array.
Typically, the individual transducer elements of the array 10 are spaced apart by one-half wavelength. This requirement is dictated by the desire for good resolution in the optical sense for the source of reflected waves being imaged. The variable delay lines could provide either continuously variable delay values or could be digitally switched between various discrete delay values. The electronic circuitry required for providing continuously variable delay values is more complicated than circuitry for providing digital switching between discrete delay values, and consequently for most practical applications switching circuitry is provided to enable digital switching between various delay values. max
For digitally switched delay lines, the criterion for good image formation is that the phase error produced at any given transducer element be less than .+-. .lambda. /8 where .lambda. is the acoustic wavelength of the ultrasonic wave in the medium through which it is travelling. To satisfy this criterion, the number of delay values (or steps) n into which the dynamic range of a given delay element can be divided should be greater than 2 N sin .theta..sub.max , where N is the total number of transducer elements in the array and .theta..sub.max is the maximum steering angle or scanning angle measured from the normal to the array. In deriving this relationship, the focal length f of the array is assumed to be large compared to the dimensions of the array, and the centers of adjacent array elements wave assumed to be separated by .lambda./2. For a typical array comprising 32 transducer elements and a maximum steering angle of 45.degree. , this criterion for good image resolution requires that there be 46 or more delay steps for each of the delay lines.
Another arrangement known to the prior art for obtaining coherent delays between the transducer elements of an ultrasonic imaging system is shown in FIG. 3, where the transducer elements 11, 12, 13, . . . , 20 are numbered as in FIG. 2. The left-end transducer element 11 is coupled to a fixed delay line 21, the right-end transducer element 20 is coupled to a fixed delay line 30, and the intervening transducer elements of the array are coupled to separate fixed delay lines 22, 23, . . . , respectively. The output signals from adjacent fixed delay lines are coupled, respectively, on either side of a variable delay element. For example, output signals from fixed delay lines 21 and 22 are coupled respectively to the two sides of delay element 40. The fixed delay lines 21, 22 and 23, . . . , 30 have differing values, as represented by the differing lengths thereof shown in FIG. 3. when it is intended to scan at an angle to the right of the normal to the array, the delay of the variable delay lines 40, 41 . . . 48 is greater than the difference of delay of adjacent fixed delay lines so that signals to and from transducer 20 are delayed more than the signals from other transducers to its left. The variable delay elements 40, 41, . . . , 48 are controlled by the controller 35. The electronic signal, which is generated by the right-end transducer element 20 when an ultrasonic wave front travelling from the right impinges thereon at an angle .theta. with respect to the normal, passes through the fixed delay line 30 to the variable delay element 48. As the wave front continues to travel after impinging the right-end transducer element 20, it impinges in succession upon each transducer element to the left of the right-end element 20. The signal generated by transducer element 19 passes through the fixed delay 29 associated therewith to the circuit line 39 where it is combined with the output of the variable delay element 48. The total delay of the signal from transducer element 20 produced by the fixed delay line 30 and variable delay element 48 is sufficiently great to allow it to combine in phase with the signal from transducer element 19 after it has passed through fixed delay line 29. The combined signals from transducers 19 and 20 are further delayed by additional variable delay elements, and combined with signals from intervening transducers. Finally, the signal contributed by the left-end transducer element 11 is coupled to the circuit line 49 at a point to the left of the variable delay element 40, and combined with the signals contributed by the preceding transducer elements.
For distantly focused ultrasonic beams, i.e., where the focal length of the array is large in comparison with the dimensions of the array, the difference in transmission time or reception time for two adjacent transducer elements is given by the expression .UPSILON. = (d/c) sin .theta. , where d is the spacing between adjacent transducer elements, c is the velocity of the ultrasonic wave in the medium through which it travels, and .theta. is the steering angle. The maximum difference in delay time between adjacent transducer elements is .UPSILON..sub.max = (d/c) sin .theta. .sub.max If the minimum value of delay for the variable delay elements is sufficiently small to be negligible, the difference in delays for adjacent fixed delay elements can be set to .UPSILON. .sub.max. The maximum required delay of the variable delay elements is then 2.UPSILON. .sub.max. The prior art required continuously variable delay elements which were set to exact delay values to match the incident wave front. The present inventor recognizes that it is possible to achieve a minimum number of delay steps for each variable delay element in order to satisfy the phase criterion stated above. Thus for the case where the minimum value of delay for the variable delay elements 40, 41, . . . , 48 is small enough to be negligible, the number of delay steps n for each variable delay element required in order to achieve good image resolution according to the criterion stated above is n = 4 sin .theta. .sub.max. In deriving this expression, it is assumed that the spacing between adjacent transducer elements is .lambda. /2.
The number of delay values required for each variable delay elements 40, 41 . . . 48, of FIG. 3 is reduced by a factor of N/2 compared to the number of delay values for each variable delay element 21, 22, . . . 30 of FIG. 2. However, the system of FIG. 3 required the addition of fixed delay elements 21, 22, 23, . . . 30. The delay required for the longest of these is at least N .UPSILON. .sub.max where N is the number of transducer elements in the array. The cost and quality of delay lines is determined by the delay-band-width product. The large number of fixed delay lines and variable delay elements required by the prior art systems illustrated by FIGS. 2 and 3 and the requirement for large delay-bandwidth products for these fixed delay lines and some of the variable delay elements contribute substantially to the system cost and complexity. The present invention provides a substantial improvement over these prior art systems by permitting a substantial reduction in the number of delay values required for each variable delay element by eliminating the need for delay values with the larger delay-bandwidth products, and not requiring any fixed delay lines.